Electrolytic method for the manufacture of dithionites

ABSTRACT

Dithionites are made electrolytically by feeding sulfur dioxide to the cathode compartment of a two-compartment electrolytic cell wherein the anode and cathode compartments are separated by a cation-active permselective membrane and in which chloride solution is being electrolyzed to chlorine and caustic. The caustic produced in the cathode compartment reacts with the sulfur dioxide therein to form sulfite which is then reduced to dithionite.

United States Patent Cook, Jr. et al.

[ Nov. 18, 1975 ELECTROLYTIC METHOD FOR THE MANUFACTURE OF DITHIONITES Inventors: Edward H. Cook, Jr., Lewiston;

Alvin T. Emery, Youngstown, both of NY.

Assignee: Hooker Chemicals & Plastics Corporation, Niagara Falls, NY.

Filed: Nov. 1, 1973 Appl. No.: 411,621

US. Cl 204/92; 204/128 Int. C13... C25B l/26; C25B 1/14; C25B 13/08 Field of Search 204/92, 296, 128

References Cited UNITED STATES PATENTS 2/1966 Beer 204/290 F 8/1970 Oloman 4/1972 McRae ..204/92 3,657,104 4/1972 Hodgdon 204/296 X 3,784,399 l/1 974 Grot 260/793 MU FOREIGN PATENTS OR APPLlCATlONS 1,045,675 10/1966 United Kingdom 204/92 Primary Examiner-4 C. Edmundson Attorney, Agent, or Firm-Peter F. Casella [57] ABSTRACT Dithionites are made electrolytically by feeding sulfur dioxide to the cathode compartment of a twocompartment electrolytic cell wherein the anode and cathode compartments are separated by a cationactive permselective membrane and in which chloride solution is being electrolyzed to chlorine and caustic. The caustic produced in the cathode compartment reacts with the sulfur dioxide therein to form sulfite which is then reduced to dithionite.

9 Claims, 1 Drawing Figure SALT ELECTROLYTIC METHOD FOR THE MANUFACTURE or DITHIONITES This invention relates to the electrolytic manufacture of dithionites. More specifically, it is of a process for making alkali metal dithionite from alkali metal chloride and sulfur dioxide, utilizing an electrolytic cell having anode and cathode compartments which are separated by a cation-active permselective membrane which, in the best embodiments of the invention, is of a hydrolyzed polymer of a perfluorinatedhydrocarbon and a fluorosulfonated perfluorovinyl ether or is a sulfostyrenated perfluorinated ethylenepropylene polymer.

The cation-permeable membranesdescribed allow a small proportion of hydroxyl ion generated at the cathode to migrate to the anolyte compartment wherein it may be converted to oxygen, thereby interfering with the efficiency of the process. However, such proportion is very low, especially considering the consumption of the hydroxyl ion in the catholyte by reaction with sulfur dioxide therein to form larger anions which cannot penetrate the cation-active permselective membrane. Thus, chloride is prevented from migrating from the anolyte to the catholyte and dithionite and sulfite ions are prevented from migrating from the catholyte to the anolyte, with the hydroxyl ion also being effectively prevented from passing through the membrane to the anolyte compartment.

Dithionites and in particular, alkali metal dithionites,

especially sodium dithionite, are useful bleaching,

agents and have been found to brighten wood pulps appreciably. Usually the dithionite employed in the past has been zinc dithionite but with the present national concern with the prevention of water and stream pollution, such as is caused by the presence of zinc ions, it has been found desirable to utilize other forms of dithionites which are less objectionable Accordingly, it has been suggested that dithionites might be made by the electrolysis of acidic solutions of 'sulfur dioxide, utilizing separating permselective membranes between anode and cathode compartments. See Pulp and Paper Magazine of Canada, Dec. 19, 1969, pages 73-78. Although such methods are feasible, the process of the present invention is considered significantly superior because it produces electrolytically the hydroxide employed to make sulfite reactant (otherwise such materials would have to be bought and chemically reacted with sulfur dioxide or other compound to make the sulfite), manufactures useful chlorine simultaneously (rather than useless oxygen) and makes a bleaching product low in chloride content and with sulfite accompanying it, which sulfite is useful in other bleach plant operations.

In accordance with the present invention a method for electrolytically manufacturing a dithionite and chlorine from sulfur dioxide and a chloride comprises feeding'sulfur dioxide to a cathode compartment of an electrolytic cell having anode and cathode. compartments, with anode and cathode therein, respectively, and a cation-active permselective membrane dividing the compartments, feeding chloride to the anode compartment thereof and withdrawing chlorine from the anode compartment and dithionite from the cathode compartment. An important advantage of this process is that it may be operated at a'pH which is about neutral, preferably about 6 to 8, in which range the dithionite is comparatively stable, so that it may be utilized commercially, in the form of the aqueous solution produced, for the bleaching of wood pulp.

Another important feature of the invention is .in the use of permselective membranes of excellent stabilities and long cell lives, so that they do not have to be repeatedly replaced. Best of these materials are the hy drolyzed copolymers of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether but the sulfostyrenated perfluorinated ethylene propylene polymers are also better than most other available cationactive permselective membranes.

The invention will be readily understood by reference to the following "description of an embodiment thereof, taken in conjunction with the drawing of an apparatus utilized in carrying out the invented process.v

In the drawing: The FIGURE is a schematic representation of an electrolytic cell and auxiliary equipment for producing dithionite by the method of this invention.

In electrolytic cell 11 outer wall 13 and bottom 14 enclose anode l5, cathode l7 and conductive means 19 and 21 for connecting the anode and cathode to sources of positive and negative electrical potentials,

respectively. Cation-active permselective membrane a 23 divides the cell volumn into anode or anolyte com partment 25 and cathode or catholyte compartment 27. An acidic aqueous solution of alkali metal halide or brine is indicated as passing into the anode compartat a level of about 22%. Chlorine, generated in theanode compartment by electrolysis of the halide solution, is taken off through line 37.

Into cathode compartment 27 is charged gaseous sulfur dioxide through line 39 and a mixture of dithionite and sulfite is removed through piping '41. Water feedto the cathode compartment is added through conduit .43 and any hydrogen or other gases which may be produced in the cathode compartment are vented via' line Instead of employing sulfur dioxide and waterfeeds,

39 and 43, respectively, and taking off all the sulfite and dithionite produced through line 41, the catholyte may be recirculated, thereby effectively increasing the volume of the catholyte compartment and allowing greater reaction time to produce the desired dithionite. A recirculation loop for the cathode compartment includes lines 51, 53 and 55, pump 57 and tank 59. Sulfur dioxide and make-up water may be added to the loop at a convenient sites, such as through lines 61 and 63 into tank 59.-The volume of the recirculation loop may be from 2 to 100,000 times that of the cathode compartment and is preferably from 10 to 1,000 times such volume. Feed lines 39 and 43 may be used in conjunction with feeds 61 and 63 or may be supplanted by them entirely.

During operation of the cell hydroxyl ions are produced at the cathode l7 and are converted to sulfite by reaction with sulfur dioxide, following which the sulfite is reduced to dithionite. Alternatively, it may be considered that the sulfur dioxide is reduced to dithionic acid which is neutralized by the hydroxyl to form the dithionite. In either case, as is illustrated schematically by arrow 47, the dithionite (and sulfite) ions do 'not penetrate the permselective membrane 23 and therefore are held in the cathode compartment 27. Similarly, halide ions, the path of which is indicated by the arrow identified by numeral 49, do not pass from the anolyte to the catholyte. However, cations, such as alkali metal ions, e.g., Na or M can go from anolyte to catholyte. A small proportion of hydroxyl ion may penetrate the membrane 23 but usually the concentration of free hydroxyl is low in thecatholyte so that the hydroxyl entering the anolyte has little effect on chloride current efficiency.

instead of adding sulfur dioxide to the cathode compartment, wherein it acts as a source of sulfite for reductiqn to dithionite and at the same time serves to help regulate the pH in the desired 6 to 8 range, sulfite may be fed to the catholyte, with other means employed for pH regulation. By such a process, although the results may not be as satisfactory as with that previously described, utilizing sulfur dioxide, dithionite can be, made. However, unless the means of reducing the alkaline pH caused by the presence of the hydroxide generated at the cathode is a chemical which produces a useful product (and which is non-interfering with the dithionite process), there will be a waste of hydroxide and possibly, even creation of a disposal problem.

The halide solution feed to the anode compartment is an aqueous solution of a water soluble metal chloride, in the usual case preferably sodium chloride. The concentration thereof is generally in the range of 200 to 320 grams/liter for sodium chloride and 200 to 360 g./l. for potassium chloride. Preferably, such solutions contain 20m 25%, more preferably about 25% of the alkali metal halide salt, as the solutions are charged to the cell or delivered to it fromthe resaturator. Generally the chloride content will be reduced to to 30% less than the original content, preferably to IO-to 20% less and normally, as with sodium chloride, the concentration of the :halide removed from the anode compartment for resaturation and return to such compartment is about 22%, as NaCl, or equivalent. Although the anolyte may be-neutral, it is often acidified so as to be of a pH in the range of about 1 to 6, preferably 2 to 4, with acidification normally being effected with a suitable acid, such as hydrochloric'acid. Water utilized to make the initial brine charge or added as make-up feed to the anode compartments and water added to the other compartments'of the cells will preferably be deionized, containing less than p.p.m. hardness, as CaCO although tap water of comparatively low hardness, e.g., under I50 p.p.m., preferably under 50 p.p.m., can be used.

The sulfur dioxide charged to the catholyte compartment or to the vessel or tank in the recirculation loop of that compartment is usually substantially pure. e.g., over 90% S0 but lower concentrations thereof. e.g.. as low as are usable because of the desirable attributes of the membrane material in preventing gas interchanges between anode and cathode compartments. Thus, the unreacted gas, e.g., nitrogen, may be removed from the cathode compartment gas space 65 through vent 45 but to avoid production of combustible 4 mixturesit will normally be desirabletoemploy sulfur dioxidefre e of uncombined oxygen.

The mixture of sulfiteand dithionite resulting from reaction of the alkali metal hydroxide produced at the cathode and the sulfur dioxide charged to the catholyte may be increased in desired dithionite content by holding it longer in the cathode compartment or recirculating the catholyte so as to effectively increase the residence time. The more sulfur dioxide charged and the greater the currentdensity the greater the quantities of sulfite and dithionite produced. Of course, the process will normally be balanced so as to produce the desired dithionite, together with sulfite, and conditions will be maintained that allow such production and result in a comparatively stable dithionite solution. Thus, the sulfur dioxide feed will be regulated to maintain the catholyte pH at about neutral, preferably about 6 to 8 and most preferably about 7. Similarly, when the charge is a sulfite, e.g., sodium sulfite, an acid such as sulfuric acid, hydrochloric acid, sulfurous acid or other acceptable acid or acidic buffer, the byproduct of which is unobjectionable in the particular process, may be charged with the sulfite to maintain the mentioned desired pH s. pH Control has the additional beneficial effect of diminishing the hydroxide concentration in the catholyte to a very small proportion, preventing all but a very minor proportion of the hydroxide generated atthe cathode from migrating through the membrane to the anolyte, where it could otherwise have been converted to oxygen, with a loss of electrical efficiency.

The effluent from the cathode compartment, the mixture of dithionite and sulfite, is usually withdrawn as an aqueous solution having a concentration of about 1 to 100 grams per liter of dithionite, frequently from about 5 to 50 g./l. and in several experiments run, within the 10 to 20 g./l. range. The proportion of sulfite produced will usually also be within the l to 100 g./l. range. The conversion of Sulfur dioxide or sulfite to dithionite will normally be at a current efficiency of from about 40 to e.g., 60%. However, the concentrations of the products can be modified, as previously mentioned, by control. of the electrolytic reaction and feed and recirculation rates, thereby controlling reaction times. Of course, changes in the reactants, pHs and the temperatures employed can also have selective effects on productions of the dithionite and sulfite, both in quantities and proportions.

To obtain the desired operation of the present cells, as described, the voltage drop is about 3 to 5 volts, preferably 3.5 to 4.5 volts; the current density is 0.1 to 2 amperes/sq. in., preferably 0.5 to 1.5 a.s.i.; and the operating temperature is 3" to 40C., preferably 5 to 20C.; with low temperatures being desirable because of the greater stability of the'dithionite at such temperatures.

The anodes employed are preferably dimensionally stable anodes of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides and mixtures thereof, on a valve metal, whereas the cathodes are preferably of stainless steel. Instead of the dimensionally stable anodes, anodes of noble metals or oxides thereof may also be employed, e.g., platinum. iridium, ruthenium or rhodium. Alternatively, other anodes resistant to the anolytes can be used, although they are not usually preferred. The anodes and cathodes may be connected to sources of electrical potential by conductive metals, such as copper, silver, aluminum, steel and iron but these materials are normally shielded from contact with theelectrolytes. Preferably dimensionally stable anode, surfaces, all on titanium or tantalum substrates, are ruthenium oxide-titanium oxide mixtures, platinum, ruthenium, platinum oxide and mixtures of ruthenium and platinum and mixtures of their oxides. A preferred dimensionally stable anode is a ruthenium oxide-titanium dioxide mixture on a titanium substrate, connected to a source of positive electrical potential by a titaniumclad copper conductor.

The cathodes employed should be resistant to the corrosive catholyte and therefore it has been found that noble metal, noble metal oxide and stainless steel cathodes are preferred. Ordinary iron, steel or graphite cathodes soon become deteriorated in use, although they may be employed for short termoperations. Of the noble metals, those previously described are satisfactory and of the stainless steels those containing small proportion of molybdenum, in addition to chromium, nickel and iron, are preferred. These include Stainless Steel Types Nos. 316 and 317. However, other stainless steels of high resistances to corrosion by the catholyte environments may also be employed, many of which may contain about 18% of chromium and 8% of nickel. The various stainless steels from which corrosionresistant anodes may be made are described in Section 24 of the Steel Products Manual, issued by the American Iron and Steel Institute in February, 1949, under the heading Stainless and'Heat-Resisting Steels. A summary of such steel formulations and corresponding type numbers is found in the Handbook of Engineering Fundamentals by Eshbach, Second Edition, published in 1952 by John Wiley & Sons, Inc., New York, page 1240 and discussions of such steels and their corrosion resistances is at page 12-39. In addition to the stainless steels, other corrosion resistant steels such as silicon steels, nickel steels, and other conductive materials resistant to corrosion may also be employed as cathode materials or surfaces.

The presently preferred cation-permselective membrane is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated or unsaturated hydrocarbons of 2'to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and most especially those of 2 to 3 carbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether which is most useful is that of the formula FSO CF CF OCF(CF )C- F OCF=CF Such a material, named as perfluoro[2- (2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying the internal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or butyl, plus rearranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluoro-lower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.

The method of manufacture of the hydrolyzed copolymer is described in Example XVII of US. Pat. No.

3,282,875 and an alternative method is mentioned in such patentsare hereby incorporated herein by reference. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetrafluoroethylene or equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular weight is indeterminate but the equivalent weight is about 900 to 1,600, preferably 1,100 to 11400 and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably 15 to 20% and most preferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant -SO F groups to SO H groups, as by treating with 10% sulfuric acid or by the methods of the patents previously mentioned. The presence of theSO H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian Pat. No. 752,427 and US Pat. No. 3,041,317, also hereby incorporated by reference.

Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sagsi 'iThe membrane is preferably joined to the backing tetr oroethylene or other suitable filaments prior 'to h K lysis, when it is still thermoplastic, and the film'of copolymer covers each filament, penetrating into the spaces between them and even around behind them, thinning the films slightly in the process, whereby they cover the filaments.

The membrane described is far superior in the present processes to all other previously suggested membrane materials. It is more stable at elevated temperatures, e.g., above 75C. It lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected to chlorine at high-cell temperatures. Considering the savings in time and fabrication costs, the present membranes are moreeconomical. The voltage drop through the membranes'is acceptable and does not become inordinately high, and it does with many other membrane materials, when the caustic concentration in the cathode compartment increases to above about 200 g./l. of caustic. The selectivity of the membrane and its compatibility with the electrolyte do not decrease detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane materials. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly as it does with other membranes when the hydroxyl ion concentration in the catholyte increased. Thus, these differences in the present process make it practicable, whereas previously described processes have not attained commercial acceptance. While the more preferred copolymers are those having equivalent weights of 900 to 1,600, with 1,100 to 1,400 being most pre- Canadian Pat. No. 849,670, which also discloses the i use of the finished membrane in fuel cells, character ized therein as electrochemical cells. The disclosures of ferred, some useful resinous membranes produced by the present method may be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell operations.

Improved versions of the above-described copolymers may be made by chemical treatment of surfaces thereof. as by treatments to modify the SO H group thereon. For example, the sulfonic group may be altered or may be replaced in part with other moieties. Such changes may be made in the manufacturing process or after production of the membrane. When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to 0.01 mm. Caustic efficiencies of the invented processes, using such modified versions of the present improved membranes, can increase about 3 to 20%, often about 5 to Exemplary of such treatments is that described in French patent publication 2,152,194 of Mar. 26, 1973 in which one side of the membrane is treated with NH to form SO NH groups. In addition to the copolymers previously discussed, including modifications thereof, it has been found that another type of membrane material is also superior to prior art films for applications in the present processes. Although it appears that tetrafluoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making satisfactory cationactive permselective' membranes for use in the present electrolytic processes it has been established that perfluorinated ethylene propylene polymer FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as 3 years or more (that of the preferred copolymers) may not be obtained, the sulfostyrenated F EPs are surprisingly resistant to hardening and otherwise failing in use under the present process conditions.

To manufacture the sulfostyrenated FEP membranes a standard FEP, such as manufactured by E. I. DuPont de Nemours & Co. Inc., is styrenated and the styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 'to is prepared and a sheet of PEP polymer having a thickness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., is dipped into the solution. After removal it is subjected to radiation treatment, using a cobalt radiation source. The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarad. After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated, preferably in the para position, by treatment with chlorosulfonic acid, fuming sulfuric acid or $0 Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about one-half hour.

Examples of useful membranes made by the described process are products at RAI Research Corporation, Hauppauge, New York, identified as 18ST12S and 16ST13S, the former being 18% styrenated and having two-third of the phenyl groups monosulfonated and the latter being 16% styrenated and having thirteen-sixteenth of the phenyl groups monosulfonated. To obtain 18% styrenation a solution of l71/2% of styrene in l7-chloride is utilized and to obtain the 16% styrenation a solution of 16% of styrene in methylene chloride is employed.

The products resulting compare favorably with the preferred copolymers previously described. giving voltage drops of about 0.2 volt each in the present cells at 8 a current density of 2 amperes/sq. in.. the same as is obtained from the copolymer.

The membrane walls will normally be from 0.02 to 0.5 mm. thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm. When mounted on a polytetrafluoroethylene. asbestos, titanium or other suitable network, for support, the network filaments or fibers will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thickness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thicknesses greater than that of the film may also be successfully employed, e. g., l. l to five times the film thickness. The networks, screens or cloths have an area percentage of openings therein from about 8 to 80%, preferably 10 to and most preferably 30 to 70%. Generally the cross-sections of the filaments will be circular but other shapes, such as ellipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to the membrane it is preferred that it be fused to it by high temperature, high pressure compression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fastened in place in a holder or support. It is preferred to employ the described backed membranes as walls of the cell between the anolyte and catholyte compartments and the buffer compartment(s) but if desired, that separating the anolyte and buffer compartments may be of conventional diaphragm material, e.g., deposited asbestos fibers or synthetic polymeric fibrous material (polytetrafluoroethylene, polypropylene). Also, treated asbestos fibers may be utilized and such fibers mixed with synthetic organic polymeric fibers may be employed. However, when such diaphragms are used efforts should be made to remove hardness ions and other impurities from the feed to the cell so as to prevent these from prematurely depositing on and blocking the diaphragms.

The material of construction of the cell body may be conventional, including concrete or stressed concrete lined with mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, TFE or other suitable plastic or may be similarly lined boxes of other structural materials. Substantially self-supporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with moldedin fibers, cloths or webs.

The processes of the invention result in the obtaining of good current efficiencies for the manufacture of chlorine and acceptable efficiencies for the conversion of sulfur dioxide or sulfite to dithionite. In preferred embodiments of the invention, when sodium chloride is utilized and sodium dithionite is made from sulfux dioxide, sulfite or mixtures thereof, the chlorine current efficiency is from 90 to 99%, usually being 94 to 97% and the conversion to sulfur dioxide or sulfite to sodium dithionite will be at a current efficiency of from about 40 to e.g., 60%. While such latter efficiencies appear to be low it should be kept in mind that useful sulfite byproduct is also produced and consequently, the overall'efficiency of the process is sufficiently high to be commercially useful. 7

The present cells may be incorporated in large or small electrochemical plants, such as those which are capable of producing dithionite bleaching solutions 9 (with accompanying sulfite) while also manufacturing from to 1,000 tons per day of chlorine or equivalent or derivative(s) thereof. In all such cases the efficiencies obtainable are such as to make the processes economically feasible. It is highly preferred, however, that the installation should be'located near to and should be used in conjunction with a groundwood or woodpulp bleaching plant so that the dithionite produced can be employed promptly as a bleach and the other chemical(s) made may also be used for pulping or bleaching purposes, without the need for shipment over long distances to ultimate consumers. If such shipment is necessary, chlorine may be liquefied beforehand to reduce its volume but the dithionite should be consumed on site or within a relatively short pipeline distance of the manufacturing plant. 7

The following examples illustrate but do not limit the invention. Unless otherwise indicated, all parts are by weight and all temperatures are in C.

EXAMPLE 1 Employing the apparatus illustrated in the FIGURE an aqueous solution of sodium dithionite is manufactured and is utilized in the bleaching of groundwood pulp. The two-compartment cells include an asbestos filled polypropylene cell box, a dimensionally stable anode of titanium having a ruthenium-titanium oxide coating (about 50% of each oxide) and a Type 316 stainless steel cathode. The anode is of titanium mesh, coated with the oxides and-connected to a source of electricity by titanium-clad copper rods. In variations of the present method essentially the same results are obtained when the internal cell walls are of polypropylene, chlorinated polyethylene or chlorinated polypropylene, the anode is of platinum, platinum-iridium alloy or platinum-titanium, the oxide coating is titanium oxide or ruthenium oxide and the cathode is of Type 317 stainless steel or other corrosion resistant material, including silicon steel.

The cation-active permselective membranes employed have a wall thickness of about 7 mils (about 0.2 mm.) and the membrane portion thereof is joined to a backing or supporting network of polytetrafluoroethylene (Teflon) filaments having a diameter of about 0.1 mm. and woven into cloth form so that the area percentage of openings therein is about The filaments are substantially circular in cross-section and the membranes mounted on them were originally flat and are fused onto the screen or cloth by high temperature, high compression pressing, with portions of the membranes actually flowing around the filaments during the fusion processes to lock onto the cloth. The described permselective membranes are obtainable from El. Du- Pont de Nemours&Company, lnc., Plastics Department, Wilmington, Delaware 19898, as XR Perfluorosulfonic Acid Membranes. The material thereof is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The by drolyzed copolymer is of tetrafluoroethylene and FSO CF CF OCF(CF )CF OCF=CF and has "an equivalent weight in the 1,100 to 1,400 range, about 1,250. The cell volumes are about equal and the electrodes are about one-fourth inch or 6.3 mm. apart, although distances from one-eighth inch to one-half inch are also feasible and in additional experiments, produce dithionite satisfactorily. I

The feed to the anode compartment is a 25% sodium chloride solution in water and the depleted anolyte is at range of 15 to 35C.) and at such a rate as to maintain the pH of the catholyte at 7,although variationsbetween 6 and 8 are experienced and are acceptable. The catholyte and anolyte temperatures are maintained in therange of 6 to 18C., to maintain production stability. Temperature is controlled by regulating the temperatu'res of the materials charged and by effecting heating or cooling in a leg or vessel of the recirculation system. An effort is made to maintain the temperature at about 6 to 12C. The volume of the recirculation loop, including the vessel and pump therein, is twenty. times that of the cathode compartment. The water and sulfur dioxide feeds to the catholyte are regulatedso as to'hold the desired temperature, the neutral pH andthe dithionite concentration for which the cell is designed. The cell is operated continuously at a potential of 3.83 volts and a current density of 1 a.sli. Chemical analysis of the product'drawn off shows'it 'to contain 14.4 grams per liter of sodium dithionite, which solu- I tion is utilized within 20 minutes to bleach groundwood pulp. Also present with the dithionite is about 5% of sodium sulfite. The efficiency for the manufacture of the dithionite is about 60%.

The solution of dithionite and sodium sulfite from the cathode compartment is continuously employed to bleach the groundwood pulp, after dilution to 1% dithionite solution. The groundwood charge is an 85:15 mixture of West Coast hemlock and balsam, the rate of application is 1.1% of sodium dithionite, on a dry pulp basis and the pulp is in a 3% aqueous slurry buffered to a pH of about 6.5 with potassium hydrogen phosphate before addition of the dithionite. A brightness increase of about 10 units is obtained at a temperature of about C. after about 30 minutes treatment. Reversion is about two units. After use of the bleach liquor the exhausted material is recovered and mixed with black liquor, which is subsequently converted to white liquor and is again employed in the pulping process.

EXAMPLE 2 The procedure of Example 1 is repeated with the exception that a steel cathode is employed and no recir-' culation leg, vessel and pump are utilized. The cell o'perates at about 12C., utilizing the same current "density and potention and producing a dithionite solution of about 14 g./1. concentration in water, accompanied by the sodium sulfite. However, after operation for a prolonged period of time, more than 24 hours, corrosion of the steel cathode causes the voltage drop to increase and efficiency of operation is lowered. Accordingly, the cathode is changed to a Type 316 stainless steel cathode and this condition is corrected, with the voltage drop diminishing and with efficiency returning to approximately that of Example 1. The dithionite solution produced is useful in bleaching groundwood pulp 7' by the method described in Example 1.

EXAMPLE 3 EXAMPLE 4 The procedure of Example 1 is followed except for the replacement of the sulfur dioxide feed to the catholyte with a feed of an equivalent proportion of sodium sulfite. The desired pH of 7 is maintained in the cathod e'compartment 'by continuous addition of sulfuric acidIJThe productionof dithionite is effected at essentially the same' rate as described for Example 1 and the product isessentially the same. The aqueous dithionite solultionobtaind is utilized for groundwood bleaching, as described in Example 1 .In variations of this procedure sodium bisulfite, sodium bisulfate and hydrochloric acid are utilized to buffer the catholyte or otherwise reduce its pH to about n'eutral (within the 6-8 range). In such cases the dithionite production is essentially the same as when sulfuric acid is employed to regulate the pH.

In further variations of these examples the voltage, current density, feed rates, electrodes, cell lining materials, recirculating procedures, rejuvenations and proportions are varied, as described in the preceding specification and in all such processes useful dithionite bleaching solution is obtained. Such is also the case when other soluble halides, e.g potassium chloride, are, employed to produce" the corresponding dithionites. Also, variations in the process by which it is made continuous or in which it is run as a batch process are effective without hurting the utility thereof. When, in the procedure of Example 1, the membrane material is replaced with the sulfostyrenated perfluorinated ethyl! ene propylene polymeric membranes, previously identified asof 18ST12S 16ST13S polymers, made by RAI Research Corporation, good dithionite production results, essentially the same, as that obtained with the PSEPVE membrane. However, after use the RAI membranes do not appear to be in as good condition as the PSEPVE membranes (they .show signs of stress and voltage drop increases somewhat), although they are far superior to other commercially available cationactive membranes, which deteriorate rapidly under the process conditions. When surface treated PSEPVE membranes are employed, such as those previously described wherein the SO H group is further chemically modified, usually to a depth of 0.001 to 0.01 mm. on the membrane, current efficiency for the dithionite production increases about and the membrane is as stable as the regular PSEPVE membrane.

The invention has been described with respect to working examples and illustrative embodiments but is not to be limited to these because it is evident that one of ordinaryskill in the art will be able to utilize substitutes and equivalents without departing from the spirit of the invention or the scope of the claims.

What is claimed is:

l. A method for electrolytically manufacturing a al kali metal dithionite and chlorine from sulfur dioxide and an alkali metal chloride which comprises feeding sulfur dioxide to a cathode compartment of an electrolytic cell having anode and cathode compartments, with anode and cathode therein, respectively, and a cation-active permselective membrane dividing the compartments, feeding an aqueous chloride solution into the anode compartment thereof, maintaining the pH in the cathode compartment in a range at which the alkali metal dithionite produced therein is stable and withdrawing chlorine from the anode compartment and alkali metal dithionite from the cathode compartment.

2. A method according to claim 1 wherein the cationactive permselective membrane is selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, and the pH of the catholyte is about 6 to 8. v

3. A method according to claim 2 wherein the permselective membrane is of a hydrolyzed copolymer of tetrafluoroethylene and FSO CF CF OCF(CF )C- F OCF=CF which copolymer has an equivalent weight of about 900 to 1,600.

4. A method according to claim 3 wherein the voltage is from about 3 to 5 volts, the current density is from 0.1 to 2 amperes per square inch, the operating temperature is from 3 to 40C. and the feed of sulfur dioxide to the cathode compartment is controlled so as to maintain a pH therein of about 6 to 8.

5. A method according to-claim 4 wherein the cell is a two-compartment cell, the membrane walls is from about 0.02 to about 0.5 mm. thick and the membrane is mounted on a network screen or cloth of filaments of a material selected from the group consisting of polytetrafluoroethylene, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to about with the filaments having a thickness of about 0.01 to about 0.5

6. A method according to claim 5 wherein the operating temperature is about 5 to 20C., the voltage is about 3.5 to 4.5 volts, the current density is about 0.5 to 1.5 amperes/sq. in., the pH is maintained in the range of 6 to 8, the chloride is fed to the anode compartment as a brine of 20 to 25% chloride content, and the dithionite is withdrawn as an aqueous solution having a concentration of about 1 to grams/liter.

7. A method according to claim 6 wherein the anode is a dimensionally stable anode of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, and the cathode is stainless steel.

8. A method according to claim 7 wherein the chloride is sodium chloride, the dithionite produced is sodium' dithionite and the anolyte is recirculated and depleted anolyte is increased in concentration to about 25% NaCl by dissolving solid sodium chloride therein.

9. A method according to claim 8 wherein the membrane copolymer equivalent weight is from 1,100 to 1,400, the membranewall thickness is 0.1 to 0.3 mm.,

- the anode is ruthenium oxide on titanium, the pH of the 

1. A METHOD FOR ELECTRICALLY MANUFACTURING A ALKALI METAL DITHIONITE AND CHLORINE FROM SULFUR DIOXIDE AND AN ALKALI METAL CHLORIDEE COMPRISES FEEDING SULFUR DIOXIDE TO A CATHODE COMPARTMENT OF AN ELECTROLYTIC CELL HAVING ANODE AND CATHODE COMPARTMENTS WITH ANODE AND CATHODE THEREIN, RESPECTIVELY, AND A CATION-ACTIVE PERMSELECTED MEMBRANE DVIDING THE COMPARTMENTS FEEDING AN AQUEOUS CHLORIDE SOLUTION INTO THE ANODE COMPARTMENT THEREOF, MAINTAINING THE PH IN THE CATHODE COMPARTMENT IN A RANGE AT WHICH THE ALKALI METAL DITHIONITE PRODUCED THEREIN IS STABLE AND WITHDRAWING CHLORINE FROM THE ANODE COMPARTMENT AND ALKALI METAL DITHIONITE FROM THE CATHODE COMPARTMENT.
 2. A method according to claim 1 wherein the cation-active permselective membrane is selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, and the pH of the catholyte is about 6 to
 8. 3. A method according to claim 2 wherein the permselective membrane is of a hydrolyzed copolymer of tetrafluoroethylene and FSO2CF2CF2OCF(CF3)CF2OCF CF2, which copolymer has an equivalent weight of about 900 to 1,600.
 4. A method according to claim 3 wherein the voltage is from about 3 to 5 volts, the current density is from 0.1 to 2 amperes per square inch, the operating temperature is from 3* to 40*C. and the feed of sulfur dioxide to the cathode compartment is controlled so as to maintain a pH therein of about 6 to
 8. 5. A method according to claim 4 wherein the cell is a two-compartment cell, the membrane walls is from about 0.02 to about 0.5 mm. thick and the membrane is mounted on a network screen or cloth of filaments of a material selected from the group consisting of polytetrafluoroethylene, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which has an area percentage of openings therein from about 8 to about 80%, with the filaments having a thickness of about 0.01 to about 0.5 mm.
 6. A method according to claim 5 wherein the operating temperature is about 5* to 20*C., the voltage is about 3.5 to 4.5 volts, the current density is about 0.5 to 1.5 amperes/sq. in., the pH is maintained in the range of 6 to 8, the chloride is fed to the anode compartment as a brine of 20 to 25% chloride content, and the dithionite is withdrawn as an aqueous solution having a concentration of about 1 to 100 grams/liter.
 7. A method according to claim 6 wherein the anode is a dimensionally stable anode of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, and mixtures thereof, on a valve metal, and the cathode is stainless steel.
 8. A method according to claim 7 wherein the chloride is sodium chloride, the dithionite produced is sodium dithionite and the anolyte is recirculated and depleted anolyte is increased in concentration to about 25% NaCl by dissolving solid sodium chloride therein.
 9. A method according to claim 8 wherein the membrane copolymer equivalent weight is from 1,100 to 1,400, the membrane wall thickness is 0.1 to 0.3 mm., the anode is ruthenium oxide on titanium, the pH of the anolyte is about 2 to about 4 and the dithionite withdrawn is in an aqueous solution having a concentration of 5 to 50 g./l. 